Car vibrates when idle with engine on with dip in rev from 700rpm to 500 RPM

I own a Honda Civic (auto transmission-1.6 l, driven 22000Km). Have serviced it 3 months ago with change in engine oil. Recently I have noticed a jarring/vibration when the engine is on but car is stationary which comes every 15 seconds or so and lasts for 1-3 seconds (as if the car is shuddering). This is associated with a drop in RPM from the baseline 700rpm to about 600 or may be occasionally 500rpm. When the shuddering stops the rpm is back to baseline. Dont feel a thing when the car is running. Still get about 9km/L in city roads. Would like your opinion please. Thanks.